Systems and methods for monitoring patient motion via capacitive position sensing

ABSTRACT

Systems and methods are described for the monitoring of patient motion via the detection of changes in capacitance, as measured using a capacitance position sensing electrode array. The changes in capacitance may be processed to determine a corresponding positional offset, for example, using a calibration data set relating capacitance to offset for each electrode of the array. The detected positional offset may be employed to provide feedback to a surgeon or operator of a medical device, or directly to the medical device for the control thereof. A medical procedure may be interrupted when the positional offset is detected to exceed a threshold. Alternatively, the detected positional offset may be employed to manually or automatically reconfigure a medical device to compensate for the detected change in position. Various configurations of capacitive position sensing devices are disclosed, including embodiment in incorporating capacitive sensing electrodes with a mask or other support structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/242,808, titled “SYSTEMS AND METHODS FOR MONITORING PATIENT MOTIONVIA CAPACITIVE POSITION SENSING” and filed on Oct. 16, 2015, the entirecontents of which is incorporated herein by reference.

BACKGROUND

External beam radiation therapy (RT) and stereotactic radiosurgery (SRS)require delivery of precisely defined x-ray beams to pre-definedlocations within the human body in order to deliver a radiation dosesufficient to kill abnormal cells. While the most common indication ofthis treatment modality is cancer, radiation can also be used to treatan array of benign indications in the brain, including vestibularschwannoma, meningioma, pituitary adenoma, arteriovenous malformation,or functional disorders such as trigeminal neuralgia or Parkinson'sdisease. SRS involves the most demanding accuracy of delivery ofradiation dose; for example, in treatment of trigeminal neuralgia, avery high dose, for example 90 Gy, is delivered in a single fraction tothe trigeminal nerve, which is only millimeters in dimension. In thesecircumstances the motion of the patient during radiation delivery mustbe reduced to near zero. Current approaches to immobilizing the patientinvolve inserting pins into the skull, which are then secured to astereotactic head frame—a practice that is invasive compared to otherpractices in radiotherapy.

For other cranial indications that are slightly less demanding withregard to spatial accuracy, in order to reduce the invasiveness of thetreatment, a “frameless” approach can be used, for example, by reducingpatient motion with a thermoplastic mask system or a bite-block.However, the motion during treatment (called intrafractional motion)with frameless systems can be significant compared to the requiredaccuracy of radiation dose delivery. Furthermore, because for manytreatment delivery systems there is no way to continuously monitor andadjust for this motion, spatial margins must be added to the treatmentvolumes during planning, which is equivalent to knowingly treatingunnecessary regions of healthy brain. Moreover, since patientpositioning usually degrades over the duration of treatment, this marginwould be larger for longer treatment delivery times. For example, Kanget al. (Med Phys 40(5), 2013) measured 3D intrafractional motion for 262patients using the Cyberknife system and concluded that margins of 2.1,3.2 and 4.2 mm would be required for treatment sessions lasting 10, 20and 30 minutes.

As noted above, therapeutic and surgical procedures, such asradiotherapy and radiosurgery, require positional monitoring whenemployed in a frameless configuration. Current methods of monitoringinclude imaging using ionization radiation (e.g. x-rays), real-timemonitoring using radiofrequency (RF) transponders implanted into thepatient, and optical monitoring of the patient's skin. Each of thesemethods have significant drawbacks, as explained below.

Imaging with x-rays adds inadvertent radiation dose to the patient. Thisis especially significant since many cranial indications are benign(e.g. acoustic neuroma, meningioma, pituitary functional disorders,trigeminal neuralgia) and patients are often young, which makesconsideration of imaging dose and consequent radiation-inducedcarcinogenesis important, e.g., compared to treating high stage cancer.There are also technical restrictions, for example many x-ray systemsare limited with regard to the temporal sampling of patient position andthe source or detector may be blocked by the dose delivery platform(e.g. linear accelerator) during the treatment delivery.

Real-time monitoring using RF transponders involves implanting smallmetallic coils into the body, making the method invasive. The approachalso involves a significant consumable cost, i.e., the transpondersthemselves, which has limited the adoption of the method in manycountries where specific remuneration does not exist. In addition, thesystem is costly and involves purchase of an RF tracking panel andacquisition system that must be coupled to the treatment deliveryplatform.

Optical monitoring of the patient skin suffers from several drawbacks,including: i) possible skin deformation, causing skin to be a limitedsurrogate for tumor position, ii) limitation of the monitored area toone eight to one quarter of the upper facial skin, iii) detection offeatures in the optical signal that are not present in the referencedata, e.g., hair, facial hair. These limitations are inherent to themethod an unlikely to be resolved (Li et al, Med Phys 38(7), 2011).

As noted above, current commercial solutions involve potential harm tothe patient (inadvertent use of x-rays), are invasive (RF transponders),or may give misleading information regarding actual tumor position(optical monitoring of skin).

SUMMARY

Systems and methods are described for the monitoring of patient motionvia the detection of changes in capacitance, as measured using acapacitance position sensing electrode array. The changes in capacitancemay be processed to determine a corresponding positional offset, forexample, using a calibration data set relating capacitance to offset foreach electrode of the array. The detected positional offset may beemployed to provide feedback to a surgeon or operator of a medicaldevice, or directly to the medical device for the control thereof. Amedical procedure may be interrupted when the positional offset isdetected to exceed a threshold. Alternatively, the detected positionaloffset may be employed to manually or automatically reconfigure amedical device to compensate for the detected change in position.Various configurations of capacitive position sensing devices aredisclosed, including embodiments incorporating capacitive sensingelectrodes with a mask or other support structure.

Accordingly, in a first aspect, there is provided a method of performingcapacitive monitoring the position of a body region during a medicalprocedure involving a therapeutic or surgical device, the methodcomprising:

positioning the body region in a reference position associated with themedical procedure, wherein at least a portion of the body region ispositioned within a sensing region of a capacitive position sensingdevice, the capacitive position sensing device comprising an array ofelectrodes, and wherein the body region is positioned without contactingthe array of electrodes;

detecting capacitance between each electrode and the body region,thereby obtaining a set of measured capacitance values;

processing the set of measured capacitance values to determine apositional offset of the body region relative to the reference position;and

controlling the therapeutic or surgical device based on the positionaloffset.

In another aspect, there is provided a method of performing capacitivemonitoring of the orientation of a body region during a medicalprocedure involving a therapeutic or surgical device, the methodcomprising:

positioning the body region in a reference orientation associated withthe medical procedure, wherein at least a portion of the body region ispositioned within a sensing region of a capacitive position sensingdevice, the capacitive position sensing device comprising an array ofelectrodes, and wherein the body region is positioned without contactingthe array of electrodes;

detecting a capacitance between each electrode and the body region,thereby obtaining a set of measured capacitance values;

processing the set of measured capacitance values to determine a angularoffset of the body region about one or more axes relative to thereference orientation; and

controlling the therapeutic or surgical device based on the angularoffset.

In another aspect, there is provided a method of performing capacitivemonitoring the position of a body region during a medical procedure, themethod comprising:

positioning the body region in a reference position associated with themedical procedure, wherein at least a portion of the body region ispositioned within a sensing region of a capacitive position sensingdevice, the capacitive position sensing device comprising an array ofelectrodes, and wherein the body region is positioned without contactingthe array of electrodes;

detecting capacitance between each electrode and the body region,thereby obtaining a set of measured capacitance values;

processing the set of measured capacitance values to determine apositional offset of the body region relative to the reference position;and

providing an alert to interrupt the medical procedure when thepositional offset exceeds a threshold.

In another aspect, there is provided a system for performing capacitivemonitoring the position of a body region during a medical procedureinvolving a therapeutic or surgical device, the system comprising:

a capacitive position sensing device comprising:

-   -   a dielectric support; and    -   an array of electrodes provided on or embedded within said        dielectric support;    -   wherein said array of electrodes is configured for capacitive        sensing within a sensing region, wherein the sensing region is        suitable for positioning at least a portion of the body region        therein, such that the body region is positionable in a        reference position within the sensing volume without contacting        said array of electrodes;

control and processing hardware operatively coupled to said capacitiveposition sensing device, wherein said control and processing hardware isconnectable to said therapeutic or surgical device for sending a controlsignal thereto, and wherein said control and processing hardware isconfigured to perform operations comprising:

-   -   detecting capacitance between each electrode and the body        region, thereby obtaining a set of measured capacitance values;    -   processing the set of measured capacitance values to determine a        positional offset of the body region relative to the reference        position; and    -   providing the control signal to the therapeutic or surgical        device based on the positional offset.

In another aspect, there is provided a system for performing capacitivemonitoring the position of a body region during a medical procedure, thesystem comprising:

a capacitive position sensing device comprising:

-   -   a dielectric support; and    -   an array of electrodes provided on or embedded within said        dielectric support;    -   wherein said array of electrodes is configured for capacitive        sensing within a sensing region, wherein the sensing region is        suitable for positioning at least a portion of the body region        therein, such that the body region is positionable in a        reference position within the sensing volume without contacting        said array of electrodes;

control and processing hardware operatively coupled to said capacitiveposition sensing device, and wherein said control and processinghardware is configured to perform operations comprising:

-   -   detecting capacitance between each electrode and the body        region, thereby obtaining a set of measured capacitance values;    -   processing the set of measured capacitance values to determine a        positional offset of the body region relative to the reference        position; and    -   providing an alert to interrupt the medical procedure when the        positional offset exceeds a threshold.

A further understanding of the functional and advantageous aspects ofthe disclosure can be realized by reference to the following detaileddescription and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the drawings, in which:

FIGS. 1A and 1B show an example capacitive position sensing devicehaving a cylindrical ring geometry, where FIG. 1A shows an exampleimplementation with a full ring, and FIG. 1B shows an alternativeexample implementation with a partial ring.

FIGS. 2A-C show several example alternative implementations ofcapacitive position sensing devices that employ multiple cylindricalportions.

FIGS. 3A and 3B plot the results of simulations showing theequipotential lines between the array of electrodes of an examplecapacitive position sensing device and an cylindrical object placedwithin the sensing region of the device, showing the effect of apositional offset of the cylindrical object on the concentration ofequipotential lines, demonstrating the varying capacitance that isspatially correlated with positional offset.

FIGS. 4A and 4B plot the results of simulations showing theequipotential lines between the array of electrodes of an examplecapacitive position sensing device and an head-shaped object placedwithin the sensing region of the device, showing the effect of apositional offset of the object on the concentration of theequipotential lines, demonstrating the varying capacitance that isspatially correlated with positional offset. The shape of the object wasderived from computed tomography data of a patient.

FIGS. 5A-C plot example calibration curves that relate known positionaloffsets to measured capacitance readings for an example cylindricaldevice. In this illustration, the measured object has been shiftedrelative to the capacitive array by seven offsets along each of thelateral, anterioposterior and superioinferior dimensions.

FIGS. 6A and 6B show (A) equipotential plots from a series of finiteelement models performed at different spatial offset values, and (B)positional offset calibration curves obtained based on the finiteelement model results for four example array electrodes.

FIG. 7 illustrates an example electrode configuration in which two ormore chevron-shaped electrodes are provided as electrodes in acapacitive position sensing device having a cylindrical configuration,such that the chevron axis is parallel to the cylinder axis.

FIGS. 8A-D show example cross-sectional profiles of several possibleconfigurations of the capacitive position sensing device.

FIG. 9 is a diagram of an example system for performing capacitiveposition monitoring during a medical procedure.

FIG. 10 is a flow chart illustrating an example method of controlling atherapeutic or surgical device based on capacitive position sensing.

FIG. 11 illustrates an example implementation in which a cylindricalcapacitive position sensing device is employed for measuring positionaloffsets of a patient's head, where a thermoplastic immobilization maskis positioned adjacent to the patient's head, and wherein thecapacitance valves are measured through the thermoplastic immobilizationmask.

FIG. 12 illustrates an example embodiment in which capacitive electrodeelements are coupled directly to, or immediately proximal to, thesurface of the mask.

FIG. 13 is a photograph illustrating an example implementation in whicha thermoplastic mask supported by an additional support structure, andwhere the capacitive position sensing device is integrated with theadditional support structure.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

As used herein, the terms “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in the specification and claims, the terms“comprises” and “comprising” and variations thereof mean the specifiedfeatures, steps or components are included. These terms are not to beinterpreted to exclude the presence of other features, steps orcomponents.

As used herein, the term “exemplary” means “serving as an example,instance, or illustration,” and should not be construed as preferred oradvantageous over other configurations disclosed herein.

As used herein, the terms “about” and “approximately” are meant to covervariations that may exist in the upper and lower limits of the ranges ofvalues, such as variations in properties, parameters, and dimensions.Unless otherwise specified, the terms “about” and “approximately” meanplus or minus 25 percent or less.

It is to be understood that unless otherwise specified, any specifiedrange or group is as a shorthand way of referring to each and everymember of a range or group individually, as well as each and everypossible sub-range or sub-group encompassed therein and similarly withrespect to any sub-ranges or sub-groups therein. Unless otherwisespecified, the present disclosure relates to and explicitly incorporateseach and every specific member and combination of sub-ranges orsub-groups.

As used herein, the term “on the order of”, when used in conjunctionwith a quantity or parameter, refers to a range spanning approximatelyone tenth to ten times the stated quantity or parameter.

As used herein, the phrases “real-time” and “near-real-time” areintended to mean that positional offset detection is performed within alatency interval that is sufficiently low such that during the latencyinterval, the motion of the patient is sufficiently small to beclinically permissible. The latency interval may vary based on clinicalapplication and context. In various embodiments, the latency intervalmay include a time range of, for example, microseconds, milliseconds, orseconds.

Unless defined otherwise, all technical and scientific terms used hereinare intended to have the same meaning as commonly understood to one ofordinary skill in the art.

Capacitive Position Sensing for Monitoring Patient Motion

In view of the drawbacks of existing patient position monitoring methodsdescribed above, it is clear that a need exists for solution thatprovides accurate and non-invasive real-time monitoring of the patientposition. Various example embodiments of the present disclosure addressthis need by providing a capacitive position sensing solution for thedetection of the motion of a body region, such as the head. In someembodiments, capacitive position sensing is employed to detect andmeasure the positional offset of a body region in real-time ornear-real-time, and the measured positional offset is employed toprovide feedback to a medical procedure.

For example, in some example embodiments, an alert is provided,displayed, or otherwise communicated to a surgeon, operator, technicianor other user when the detected positional offset exceeds a pre-selectedthreshold. In another example implementation, the alert is provided as acontrol signal that is employed to interrupt the operation of a medicaldevice, such as a therapeutic or surgical device. In some exampleembodiments, the detected positional offset (e.g. caused byintrafractional motion) may be communicated to a surgeon, operator,technician or other user in order to allow the patient to beappropriately re-positioned. In another example embodiment, the detectedpositional offset may be provided to a medical device, such as atherapeutic or surgical device, so that the medical device can bereconfigured to compensate for the positional offset. Examples oftherapeutic and surgical devices include, but are not limited to,radiotherapy devices, radiosurgical devices, and robotic therapy orsurgical devices.

According to various embodiments, capacitive position sensing isperformed via a capacitive position sensing device that includes anarray of electrodes provided on or within a dielectric support, whereeach electrode is electrically addressable for the detection ofcapacitance. The array of electrodes are capable of sensing thecapacitance within a sensing region adjacent to the electrode array,such that when at least a portion of a body region (i.e. a body part oranatomical part, such as the head) is placed within the sensing region,the capacitance between the electrode array and the portion of the bodyregion is measurable when a suitable current or voltage is appliedbetween respective electrodes of the electrode array and the body.

Unlike some of the aforementioned position monitoring devices known inthe art, the devices, systems and methods of the present disclosure donot require direct contact with the patient (apart from optionallygrounding the patient, as described below), thereby providing anon-invasive solution. Various embodiments disclosed herein also providecapacitance position detection without radiation in a passiveconfiguration, apart from the application of electric fields, whileremaining independent of the treatment delivery platform. Furthermore,as described below, various embodiments of the present disclosure areadaptable to common mask-type immobilization systems.

The position monitoring methods of the present disclosure may not be assensitive to skin position or deformation as some of the known positionmonitoring devices described above. For example, in the case of themonitoring of the head of a patient, the capacitance methods disclosedherein may be sensitive to the patient bulk (e.g. entire cranium), asopposed to merely the skin (which can be a poor surrogate for theposition of an internal organ or internal pathological structure, suchas a tumor).

Furthermore, the systems and devices of the present disclosure may bebeneficial in providing position monitoring at a lower cost that some ofthe aforementioned systems known to those skilled in the art.

Referring now to FIG. 1A, an example capacitance position sensing device100 is shown, where the example capacitance position sensing device 100includes a cylindrical dielectric support 120 having an array ofcapacitance sensing electrodes 110 distributed thereon. Each capacitancesensing electrode 110 (also referred to herein as a “conductive pad”) iselectrically addressable, for example, via electrical contacts 130provided on the outer surface of dielectric support 120.

In the example electrode configuration shown in FIG. 1A, the capacitancesensing electrodes 110 are capable of sensing the capacitance associatedwith objects residing within sensing region 140. For example, the headof a patient, or a portion thereof, may be inserted within region 140for capacitance-based position sensing. FIG. 1B illustrates analternative example configuration involving a half-cylinder capacitanceposition sensing device 101, suitable for detecting positional offsetsof a the head of a patient 150 based on capacitance detection viaelectrodes 110. The capacitance position sensing device could beemployed to provide two or three-dimensional positioning information,depending on the orientation of the array electrodes.

It will be understood that electrical contact may be made with theelectrodes 110 according to a wide range of methods, such as, forexample, soldering or wire bonding, or via electrical connectors. Thecapacitance sensing electrodes are shown in FIG. 1A as being evenlydistributed in an array, but it will be understood that the electrodes110 need not be uniformly distributed, and that the configuration shownin FIG. 1A (and other figures of the present disclosure) are mereexamples.

The suitable material composition and thickness of the capacitancesensing electrodes 110 and dielectric support 120 will vary depending onthe clinical application, as described below.

Other Example Configurations of the Capacitance Position Sensing Device

It is noted that the capacitive position sensing device is not limitedto a ring-type spatial configuration, and that a wide variety ofelectrode spatial configurations, and associated dielectric supportstructures, may be employed. For example, the dielectric support 120need not have a circular shape, and can take on a wide variety of curvedshapes (e.g. an oval shape), or a shape based on a plurality of flatsegments (e.g. a rectangular shape), for example. In some embodiments,the spatial configuration of the array of electrodes may be determinedbased on that of an existing patient support structure, as described infurther detail below.

Referring now to FIG. 2A, an alternative example embodiment is shown inwhich a second cylindrical segment 121, having a second array ofcapacitance sensing electrodes (not shown), is provided at an anglerelative to the first dielectric support 120. Such an embodiment iscapable of capacitance-based position sensing in three-dimensions,enabling, for example, the detection of positional offset in the z (i.e.cranial-caudal) dimension.

Although the first and second dielectric supports are shown in anorthogonal configuration, it will be understood that the relative anglebetween the two cylinder axes need not be 90 degrees, and that two ormore additional cylindrical segments may be included. Examples of suchan embodiment are shown in FIGS. 2B and 2C, where multiplehalf-cylindrical segments 121, 122 and 123 are shown in examplecapacitive position sensing device 103 of FIG. 2B, and multiplecylindrical segments 124, 125 and 126 are shown in the examplecapacitive position sensing device 104 of FIG. 2C. The exampleembodiment of FIG. 2C also shows the capacitive position sensing device104 being supported by a patient support 160.

As noted above, the cylindrical embodiments shown in FIGS. 1A-B andFIGS. 2A-C are merely provided as illustrative embodiments, and a widevariety of electrode spatial configurations are envisioned withoutdeparting from the scope of the present disclosure. For example, inanother example embodiment, the electrodes may be provided on adielectric support having a spherical shape (i.e. the inner surface ofthe capacitive position sensing device is shaped as a portion ofsphere). Moreover, although the preceding example embodiments illustratecapacitive position sensing devices that at least partially surround orhouse the body region, in other example embodiments, the capacitiveposition sensing device may be configured to be placed adjacent to thebody region without surrounding the body region. It is expected that asuitable spatial configuration of the electrodes will vary depending onthe type of medical procedure and the type of body region.

Calculation of Position from Capacitive Measurements

A conventional capacitor consists of two conductors, separated by adielectric material. Each capacitance sensing electrode 110 (conductivepad) in the capacitance position sensing device acts as a single plateof a capacitor; when the body region of a patient or subject isintroduced into, or adjacent to, or proximal to, the capacitanceposition sensing device, such that at least a portion of the body regionlies within the sensing region, the intervening air acts as thedielectric and the patient acts as the second conductor. Sincecapacitance is defined asC=Aε/dwhere A is the area of the conductor, ε is the permittivity of thedielectric and d is the distance separating the conductors, variationsin the distance between the patient and the electrodes of the capacitiveposition sensing device will produce corresponding variations in themeasured capacitance in an inverse relationship.

This position-dependence of the measured capacitance is illustrated inFIGS. 3A and 3B, which show the results of finite element analysis (FEA)simulations in the example case of a cylindrical body 170 positionedwithin the sensing region of a cylindrical capacitive position sensingdevice (such as the example embodiment shown in FIG. 1A). In FIG. 3A,the equipotential lines surround the grounded cylinder 170 and aresymmetrically distributed with respect to the electrode array, such thatthe measured capacitance between each electrode and the cylinder is thesame. For example, the density of equipotential lines between electrode110A and the cylinder 170 is the same as the density of equipotentiallines between electrode 110B and cylinder 170. FIG. 3B shows the changesin the equipotential lines that are caused by a positional offset of thecylindrical body 170. The positional offset has removed the symmetry,such that the density of equipotential lines now differs among theelectrodes. For example, the density of equipotential lines is muchhigher between electrode 110B and the cylinder 170 than betweenelectrode 110A and the cylinder 170. The positional offset is thereforeencoded into the spatially-dependent capacitance values. FIGS. 4A and 4Bshow similar results for a body in the shape of a human head, where theshape was obtained from computed tomography of a subject.

The capacitance can be measured according to a wide range of capacitancedetection methods known to those skilled in the art. Example methods ofcapacitance detection include, but are not limited to: applying a knowncharge to a pad and measuring potential; applying a known potential to apad and measuring charge; constructing an oscillating circuit wherebythe frequency of that circuit depends on capacitance, and measuringfrequency. In another example implementation, a capacitance bridge maybe employed to measure an unknown capacitance value. In one exampleembodiment, the electrodes of the array can be interrogatedsimultaneously for the detection of capacitance. In another exampleembodiment, the electrodes of the array can be interrogated sequentiallyfor the detection of capacitance. For example, in the latter case, whena given electrode is not being interrogated, it could be grounded.Various capacitance detection devices are presently commerciallyavailable, such as the Freescale Semiconductor model MPR03X ProximityCapacitive Touch Sensor Controller.

In the examples described below, each conductive pad in the ring wasconnected to a capacitance sensor (e.g., MPR121, FreescaleSemiconductor, Inc.), providing a set of capacitance measurements overthe surface of the ring. By combining the measurements mathematically,the x and y position of the patient can be determined.

By combining and mathematically processing the set of capacitance valuesmeasured from the capacitive pads of the capacitive position sensingdevice, the positional offsets of the body region can be determined.Positional offsets indicate the spatial deviation of the patient from areference position (baseline position). The baseline position can bedetermined, for example, through image guidance used routinely in theprocedure, or via a positioning structure (e.g. a mask) against whichthe body region is initially positioned.

As described below, the positional offsets may be provided in multipledimensions, such as two dimensions or three dimensions. For example, intwo dimensions for the ring-type example embodiments shown in FIGS. 1Aand 1B, Δx and Δy are the lateral and anterioposterior positionaloffsets, which may be determined using

${\Delta\; x} = {\sum\limits_{i = 1}^{n}\;{k_{i}\cos\;\Theta_{i}}}$ and${\Delta\; y} = {\sum\limits_{i = 1}^{n}\;{k_{i}\sin\;\Theta_{i}}}$where C_(i) is the capacitance of the i^(th) pad, and Θ_(i) is the anglebetween the horizontal axis and the centroid of the i^(th) conductivepad. In the above equations, k is a capacitive pad-specific calibrationfunction that may account for, for example, individual pad response, orthe variation in sensitivity between pads for a given positional offsetof the patient. For example, since distance between the capacitiveplates varies as 1/C, this function may be expressed in the form

$k_{i} = {R_{i}\left( {M_{i} - \frac{1}{C_{i}}} \right)}$where R_(i) and M_(i) are constants determined empirically.

The sampling frequency of the detection of capacitance values may beselected to be sufficiently high in order to detect patient motion inreal-time or near-real-time (e.g. intrafractional motion). It will beunderstood that a minimum sampling frequency may depend on a wide rangeof factors, including, but not limited to, the type of medicalprocedure, patient-specific aspects of the medical procedure (e.g. thesize and geometry of a tumor), and the nature (e.g. amplitude andfrequency range) of motion associated with a given patient. In someexample embodiments, the sampling frequency of a given electrode may beselected to be greater than 10 Hz, greater than 100 Hz, greater that 1kHz, greater than 10 kHz, or greater than 100 kHz. In some exampleembodiments, the frequency of positional offset detection, based on theinterrogation of all electrodes and the processing of the set ofcapacitance values to infer positional offset, may be greater than 10Hz, greater than 100 Hz, greater that 1 kHz, greater than 10 kHz, orgreater than 100 kHz.

Calibration Methods

In some embodiments, calibration data is employed when processing themeasured capacitance values to infer the spatial offset(s). In oneexample embodiment, the relationship between positional offset of thepatient and capacitive measurements may be determined through anempirical calibration routine. For example, with the body region of thepatient provided in a stationary configuration, the practitioner mayapply known offsets to the capacitive position sensing device, forexample, via manually or automated means. Such an embodiment avoidsshifting the patient, and spatial offsets are equivalent to motion ofthe patient but opposite in direction. Alternatively, the body regionmay be translated by known amounts relative to the capacitive positionsensing device.

For each known offset applied, all capacitive signals C_(i) are read andrelated to the offset introduced. This may be repeated sequentially forthe anterioposterior, lateral and superioinferior dimensions orcombinations thereof. The result of this process is a set of (offset,capacitive reading) pairs. This calibration data set may be stored inseveral forms, for example, as a discrete data set that can beinterpolated during subsequent processing, or parameterized, for examplethrough curve fitting.

An example of parametric curve fitting of a subset of the calibrationdata set is shown in FIGS. 5A-C, where example calibration curves thatrelate known positional offsets to measured capacitance readings areplotted for an example cylindrical device. In this illustration, themeasured object has been shifted relative to the capacitive array byseven offsets along each of the lateral, anterioposterior andsuperioinferior dimensions. For each offset, the capacitance reading foreach of each capacitive pad is measured. For each pad, the relationshipbetween capacitance and offset can be parameterized, for example,through curve fitting. FIGS. 6A and 6B show results from a second set ofFEA simulations, in which the dependence of lateral offset of acylindrical body is plotted for four selected electrodes, with eachelectrode exhibiting a unique dependence of capacitance on spatialoffset.

The parameterization yields a series of expressionsΔx _(i) =F _(x)(C _(i)),Δy _(i) =F _(y)(C _(i)) andΔz _(i) =F _(z)(C _(i))where (Δx, Δy, Δz) are the three-dimensional positional offsets of thebody region (e.g. cranium) and F_(x), F_(y) and F_(z) are therelationships between positional offset to capacitance readings C_(i)previously established.

Having obtained the calibration data set, the spatial coordinates of thebody region may be determined based on a set of capacitance values, forexample, by comparing the set of measured capacitance values to thecalibration data set and selecting spatial offsets that produce the bestmatch between the calibration data and the measured set of capacitancevalues.

In one example embodiment, one set of (Δx, Δy, Δz) coordinates may beseparately calculated for each capacitance sensing electrode (capacitivepad). The algorithm calculating positional coordinates may report, forexample, the mean spatial offset obtained from the set of capacitancesensing electrodes, and optionally one or more additional statisticalmeasures, such as median and standard deviation. The per-electrodespatial offsets may also be compared in order to identify, andoptionally discard, any spatial offsets that appear to be outliers (e.g.based on comparing the offset values for a given electrode to theaverage or standard deviation of the offset values for the remainder ofelectrodes). Alternatively, the algorithm may determine which pad ismost sensitive to the offset which has occurred, given, for example, bythe absolute offset of the capacitive reading from baseline or curvatureof the parameterization for that measured capacitance value, and maycalculate the positional offsets based on that capacitance readingalone. In yet another example embodiment, the determination of thespatial offset may be determined by applying weighting factors to theper-electrode spatial offset values when computing the net spatialoffset, such that the spatial offset values corresponding to electrodeswith the highest sensitivity (e.g. closed proximity) receive the highestweights.

This calibration procedure may be made practical and convenient for theclinical application. If the array is shifted by automated means, e.g.,by actuators, this calibration may be performed in a time efficientmanner. In addition, the calibration routine may be performed well inadvance of the procedure and stored for later use, or performed in situimmediately prior to the procedure. Furthermore, an advantage of thisempirical calibration approach is that it may be established on a perpatient basis, such that the calibration data set is a per-patientcalibration data set. Accordingly, if inter-patient variations existwith regard to the relationship between capacitance and positionaloffset, this may be accounted for during the calibration procedure.

In another example embodiment, the calibration data set may begenerated, at least in part, via simulations involving a mathematicalmodel. For example, FIG. 3A shows the result of a Finite ElementAnalysis (FEA) illustrating equipotential lines within a ring-geometrycapacitive array, with a grounded cylinder 170 approximating the patientat the center of the ring. In the FEA, for any position of the patientwithin the capacitive array, the capacitance of each pad can becalculated, since both the charge and potential of the capacitive padsare returned by the analysis. If the simulated patient is then shiftedas shown in FIG. 3B, new values of the capacitance of each pad can becalculated. By repeating this process over a range of positional offsetvalues, patient position and capacitance values can be related and usedin the generation of the calibration data set.

In some cases, the measured capacitance may depend significantly on thegeometry of the patient, and a cylindrical approximation of the bodyregion may not be sufficient. In such cases, a more realistic anatomicalmodel may be employed. For example, an approximate model may begenerated based on an atlas or based on a set of patients, or aper-patient model may be employed. For example, in radiotherapy orradiosurgery, it is standard practice to perform Computed Tomography(CT) imaging of the patient prior to the procedure for treatmentplanning purposes. In this application, CT image data may be used togenerate a model within the FEA. An example of equipotential linesgenerated with a patient specific model is shown in FIG. 4A. Asexplained above, by introducing positional offsets of this simulatedpatient, as illustrated in FIG. 4B, a capacitance-to-positioncalibration may be realized. This approach is advantageous since it ispatient-specific, may be conducted off-line well in advance of thepatient's procedure, and uses the planning CT data which is availableduring the process of radiotherapy or radiosurgical treatment planning.

In another example embodiment, the spatial offset values may bedetermined without the need for a priori calibration data. Prior toinitiating the medical procedure, the body region is positioned in thereference position. Capacitive sensing is then performed with thecapacitive position sensing device, and the capacitive position sensingdevice is dynamically actuated, with a set of motors, in order tomaintain, or best approximate, the initially measured set of capacitancevalues. The signals provided to the motors, and/or a measured positionalchange of the capacitance position sensing device, may be employed toinfer the positional offset of the body region.

Other Design Variations

It will be understood that the number of electrodes in the array ofelectrodes of the capacitive position sensing device may vary fromrelatively few electrodes (e.g. our four conductive pads; left/right andanterior/posterior) to a much larger number of electrodes (e.g. 10, 20,50, 100 or 200, or more). The number of electrodes needed may depend,for example, on the type of medical procedure, the size and geometry ofthe body region, and the desired response time of the device. Anadditional design consideration is the geometry of the electrodes, whichcan be solid or, for example, hatched.

In one example implementation involving a cylindrical geometry,electrodes could be arranged as a set of two or more adjacent chevronsthat are spatially nested, where the axis of the chevrons is directedalong the dimension that is perpendicular to the x-y plane of thecylinder array, as shown in FIG. 7. By comparing signal from the firstand second chevron (or among more than two), information regardingz-position could be obtained. This could also be done with adjacenttriangles, or other related geometrical shapes having vertices.

Additional optional design variations include the inclusion ofelectrical shielding and/or field focusing features. For example,shielding may be provided between pads or the entire array may beshielded.

FIGS. 8A-D show possible cross-sections of the device, where FIG. 8Ashows an example embodiment in which capacitive sensing electrode 110 isprovided on the back of dielectric support 120. FIG. 8B shows analternative example embodiment in which an electromagnetic shieldinglayer 180 is included on the top surface of the dielectric support 120,as it may be advantageous in some applications to add an electromagneticshielding layer to minimize the effects of stray electromagnetic fieldsin the vicinity of the device. In addition, for some applications it maybe beneficial to add a field-defining layer 190, as shown in FIG. 8C, inorder to focus electric fields produced by charge on the capacitive padsto the inner region of the capacitive array. For example, an electricpotential equal or close to that of the capacitive pads may be appliedto the field-defining layer. As shown in FIG. 8D, the device may also beconstructed to include both the electromagnetic shielding layer 180 andthe field-defining layer 190. The layers of the device may beconstructed of rigid or flexible materials, depending on theapplication. For example, the electrodes and dielectric backing may beprovided as a flexible substrate, such as a flex circuit.

As noted below, in radiosurgical or radiotherapeutic applications inwhich the capacitive positioning sensing device is positioned such thatthe radiation beam passes through the device when irradiating the bodyregion, the dielectric support 120, electrodes 110, and electromagneticshielding layer 180 and field-defining layer 190 (when present), shouldintroduce minimal attenuation of the radiation beam, such that asufficient flux and/or intensity of the beam is delivered to the bodyregion.

Example System Configuration for Performing Capacitive PositionMonitoring During a Medical Procedure

Referring now to FIG. 9, an example system is shown for performingcapacitive position monitoring during a medical procedure. The examplesystem includes a capacitance position sensing device 100, placed inclose proximity to the body region of the patient for monitoring thepositional offset thereof (in the illustrated example, the body regionis the head).

Capacitive position sensing device 100, which includes an array ofcapacitive sending electrodes as described above, is interfaced withcapacitance detector 310, such that each electrode is separatelyaddressable. The capacitance detector 310 is configured to detect thecapacitance between each electrode of the array of electrodes of thecapacitive positioning sensing device 100 and the body 150. Capacitancedetector 310 may employ any suitable method of capacitance detection,including, but not limited to, any of the method described above. Asshown in the figure, the capacitance detector 310 may provide areference or ground connection (e.g. electrode) 312 that is brought intoin electrical communication with the patient's body 150, directly orindirectly. Although, in some implementations, the patient's body may begrounded, it will be understood that grounding may not be required inorder to measure capacitance.

Capacitance detector 310 is interfaced with control and processinghardware 200, which is receives sets of capacitance values fromcapacitance detector 310, and optionally controls capacitance detectorsuch that set of capacitance values are measured at prescribed timeintervals. As shown in the example embodiment illustrated in FIG. 9,control and processing hardware 200 may include a processor 210, amemory 220, a system bus 205, one or more input/output devices 230, anda plurality of optional additional devices such as communicationsinterface 260, display 240, external storage 250, and data acquisitioninterface 270.

The present example methods of performing capacitive position sensingcan be implemented via processor 210 and/or memory 220. As shown in FIG.9, the positional offset is calculated by control and processinghardware 200, via executable instructions represented as capacitanceposition processing module 290. The control and processing hardware 200may include and execute instructions for planning and navigation of amedical or surgical procedure, as shown by planning and navigationmodule 280.

As shown in FIG. 9, control and processing hardware 200 may beinterfaced with a medical device, such as therapeutic or surgical device350, for providing control signals thereto. In one exampleimplementation, control and processing hardware 200 is programmed with apre-selected offset threshold, such that when the positional offsetexceeds the offset threshold, a control signal is provided to thetherapeutic or surgical device 350 to interrupt its operation (e.g.turning off a radiation beam, or interrupting beam delivery via aninterlock). In one example implementation, two or more thresholds may beprovided, where each threshold corresponds to a different spatialdirection or dimension. In another example implementation, thepositional offsets are provided to the therapeutic or surgical device350 such that the therapeutic or surgical device can compensate forchanges in the position of the body region. In another exampleimplementation, the positional offsets are provided to the therapeuticor surgical device or to a navigation or planning system associated withthe device, such that the location of one or more target locationsassociated with the medical procedure are dynamically updated. Inanother example implementation, the positional offsets are employed tocontrol the position and/or orientation of the therapeutic or surgicaldevice to compensate for changes in the position of the body region.

FIG. 10 provides a flow chart illustrating an example method ofcontrolling the operation of a therapeutic or surgical device inresponse to a positional offset detected with a capacitive positionsensing device, where the operation of the device is interrupted whenthe measured offset exceeds a pre-selected threshold. In step 400, a setof capacitance values are detected from the electrodes of thecapacitance sensing device. These capacitance values processed andcompared, in step 405, to a calibration data set. Positional offsetvalues are then obtained, in step 410, from the calibration data set,where the positional offset values best correspond to the measured setof capacitance values. The positional offset values are then compared toa threshold in step 415, and if the positional offset values offsetexceed the threshold (see step 420), then a control signal is sent tointerrupt operation of a therapeutic or surgical device, as in step 425.

In alternative embodiments, the control and processing hardware 200provide feedback to an operator, surgeon, technician, or other user,such that the user can intervene and control the operation of thetherapeutic or surgical device based on the detected positional offset.In one example implementation, the control and processing hardware 200is programmed with a pre-selected offset threshold, such that when thepositional offset exceeds the offset threshold, an alert is communicated(e.g. via an audible alarm and/or visual indication on a displaydevice). In another example implementation, the detected positionaloffsets may be communicated to the user (e.g. via information displayedon a user interface), such that the therapeutic or surgical device, or aplanning or navigation system associated with the device, can bereconfigured to compensate for changes in the position of the bodyregion.

The functionalities described herein can be partially implemented viahardware logic in processor 210 and partially using the instructionsstored in memory 220. Some embodiments may be implemented usingprocessor 210 without additional instructions stored in memory 220. Someembodiments are implemented using the instructions stored in memory 220for execution by one or microprocessors. Thus, the disclosure is notlimited to a specific configuration of hardware and/or software.

It is to be understood that the example system shown in FIG. 9 is notintended to be limited to the components that may be employed in a givenimplementation. For example, the system may include one or moreadditional processors. Furthermore, one or more components of controland processing hardware 200 may be provided as an external componentthat is interfaced to a processing device. For example, as shown in thefigure, capacitance detector 310 may be included as a component ofcontrol and processing hardware 200 (as shown within the dashed line),or may be provided as one or more external devices.

While some embodiments can be implemented in fully functioning computersand computer systems, various embodiments are capable of beingdistributed as a computing product in a variety of forms and are capableof being applied regardless of the particular type of machine orcomputer readable media used to actually effect the distribution.

At least some aspects disclosed herein can be embodied, at least inpart, in software. That is, the techniques may be carried out in acomputer system or other data processing system in response to itsprocessor, such as a microprocessor, executing sequences of instructionscontained in a memory, such as ROM, volatile RAM, non-volatile memory,cache or a remote storage device.

A computer readable storage medium can be used to store software anddata which when executed by a data processing system causes the systemto perform various methods. The executable software and data may bestored in various places including for example ROM, volatile RAM,nonvolatile memory and/or cache. Portions of this software and/or datamay be stored in any one of these storage devices. As used herein, thephrases “computer readable material” and “computer readable storagemedium” refers to all computer-readable media, except for a transitorypropagating signal per se.

Potential Applications

In concept the device could be used in any application where a rapid(e.g. real-time or near real-time), non-contact, non-invasive readout ofposition of the head (e.g. cranium) or other body region (i.e. portionof the human anatomy; body part) is needed.

The present example systems and methods may be applied, for example, tomedical procedures that employ photon energy beams, particle beams,and/or high-intensity focused ultrasound. In such beam delivery basedapplications, the capacitive position sensing device may be made frommaterials that permit the passage of the beam with suitable transparency(e.g. greater than 80%, greater than 90%, greater than 95%, greater than99%, or greater than 99.5% flux and/or intensity). In order to meet suchconstraints, the electrodes can be formed from a thin metallic layer,such as a thin layer of conductive paint or a thin metallic layerdeposited on a substrate such as aluminized Mylar® or Kapton®. In oneexample implementation, the electrodes may be formed from a materialhaving a conductive exceeding 32 ohms for a 10 mm×10 mm section. Theelectrodes may be connected via a conductor to capacitance detector(e.g. a circuit which measures capacitance). This conductor may becomprised of a shielded cable or shielded trace (e.g. formed from a thinmetal layer or conductive paint), but it must not significantlyattenuate the radiation therapy beam. In some example embodiments,adjacent capacitive pads in the array may be separated by dielectric (asin the present embodiment) or alternatively by additional conductiveelements at a different potential from the capacitive pads, for example,at 0V (such as in the “parallel fingers” configuration). If theadditional conductive elements are provided at a similar potential asthe capacitive sensing electrodes, the electric field may be confined bythe presence of the additional conductive elements.

The substrate 120 may consist of a material that is durable but alsointroduces minimal attenuation of the radiation beam, for example, athin-walled, hollow or close-cell foam-filled carbon-fiber shell.

In other example embodiments, the capacitive positioning sensing devicemay include an aperture that permits the delivery of the beam to thebody region.

Example applications of the embodiments disclosed above includestereotactic radiosurgery and radiotherapy (e.g. cranial surgery). It isto be understood, however, that the radiosurgical and radiotherapeuticapplications are merely provided as example applications, and that thesystems and methods described herein can be applied to many otherapplications, such as, but not limited to, navigated surgicalprocedures, robotic surgery and imaging procedures. The methods providedherein may additionally or alternatively be employed to pre-operativelyscreen patients for surgical interventions in order to determine whetheror not a given patient is likely to be capable of maintaining apositional range during a time duration associated with a medicalprocedure.

Radiosurgery and Radiotherapy Applications

Example applications of the embodiments disclosed above includestereotactic radiosurgery and radiotherapy. Such applications mayinvolving the following constraints: absence of highly-attenuatingmaterials in the paths of the incoming radiation beams, which wouldperturb the treatment delivery; capacity to read-out the position of thepatient at high temporal frequency (e.g., many times per second);measurement of patient position in two or three dimensions; the abilityto monitor the position of the cranium as a whole, rather than, e.g.,just the skin; and absence of unwanted ionizing radiation (e.g., unlikeimaging using x-rays). The embodiments disclosed herein may be adaptedto meet these requirements. For cranial indications, the device couldintroduce the options of i) eliminating the invasive head-ring,replacing with the head-ring with mask immobilization combined withreal-time monitoring, or ii) monitoring of the cranium for existingtreatments that employ rigid mask immobilization. Given the use ofextremely thin conductive elements, the device does not introducesignificantly attenuating materials around the patient, meaning that thedelivery of radiation is not perturbed. This would not be the case forother detection systems, e.g., pressure sensors or ultrasonictransducers.

Use of Capacitive Position Sensing Device with Mask

In some embodiments, a mask, or other restraining or immobilizationdevice having a dielectric structure, may be placed between the bodyregion and the capacitive position sensing device. It will be understoodthat the phrase “mask”, as used herein, refers to a restraining orimmobilization structure that is placed adjacent to the region of thebody. For example, the mask 500 may be a thermoplastic mask configuredto restrain the head of the patient during a medical procedure, asillustrated in FIG. 11. If a patient is positioned within a such athermoplastic mask, the capacitance position sensing device will nottrack the mask position (which is not of interest and may be a poorsurrogate for patient position due to the motion of the patient relativeto the mask), and instead tracks the position of the patient within themask. Such a mask may be employed to define the reference position, suchthat the positional offsets are measured relative to the mask, therebytracking patient motion relative to the mask. The mask may be employedduring calibration measurements, or included in calibration simulations,in order to account for the presence and effect of the dielectric on themeasured capacitance values.

In one example embodiment, the mask may define the dielectric substrateof the capacitance position sensing device, or at least a portionthereof, such that the electrodes are provided on or within the mask.For example, the electrodes may be embedded within the mask or providedon the inner or outer surface of the mask. In one example embodiment,the electrodes may be formed on, or attached to, the outer surface ofthe mask, optionally via an additional supporting dielectric structurethat contacts the mask. An example of the latter is shown in FIG. 12,where one or more flexible dielectric structures 510, 520 (e.g. tape) isemployed to attach (mechanically couple) the capacitive sensingelectrodes 530 directly to, or immediately proximal to, the surface ofthe mask 500. The use of such flexible structures (e.g. tape or flexiblebands) could replicate any of the geometries shown in the previousfigures in order to provide detection of motion in x, y and zdimensions. The flexible bands could be adhesive, or individuallyadjustable in order to conform to various mask shapes.

Referring now to FIG. 13, some existing patient positioning systemsinvolve additional structural support surrounding the thermoplasticmask. For example, the device shown in FIG. 13 employs a framelesssupport structure 600 made of low-density carbon fibre (a “framelessarray”). Reflective fiducial markers 610 (spheres on anterior surface inimage) are shown as being provided in order to track the position of thesupport structure 600, but these fiducials do not track the position ofthe cranium itself.

In one example embodiment, the support structure 600 (optionally withoutthe fiducial markers 610) could be employed to support the array ofelectrodes of the capacitive position detection system. For example,suitable positions for the inclusion of capacitive position sensingelectrodes on the support structure 600 are overlaid on the image shownin FIG. 13 in dashed lines (see labels 602, 604, 606 and 608). In oneexample implementation, the electrodes could be provided on thindielectric support layers, which could be attached to, for example, theinner surface of the carbon fibre frame 600.

Capacitive Sensing of Angular Offset

In some embodiments, the capacitive position sensor device may beemployed to detect changes in angular orientation in addition to, or inalternative to, the position sensing embodiments described above. Forexample, the calibration data set described above may be configured toinclude angular-dependent calibration data in addition to, on insteadof, the positional data. In cases in which the calibration data includesboth position and angular calibration data, both the positional andangular offset may be monitored, thereby providing for the monitoring ofsix degrees of freedom in selected embodiments.

Such embodiments may be useful to correct for misalignments in systemsthat are equipped with robotic compensation that can remove x, y, z aswell as roll, pitch and yaw errors. Similar to the idea of shifting thearray by known amounts and reading out capacitance values, the arraycould be rotated about the three axes by known amounts. It is noted thatin some embodiments, the angular offset detection may be performed overa subset of the three angular axes. Alternatively, in some cases, smallangular perturbation may be approximated by positional offsets. Forexample, the pitch motion is the most common in cranial patients (i.e.nodding) but the centre of rotation is quite inferior, so thistranslates into displacement in both the anterioposterior andsuperioinferior axes.

EXAMPLES

The following examples are presented to enable those skilled in the artto understand and to practice embodiments of the present disclosure.They should not be considered as a limitation on the scope of thedisclosure, but merely as being illustrative and representative thereof.

Example Capacitive Position Sensing Device

The example capacitive position sensing device shown in FIG. 1A,consisting of a very low density ring with an array of conductive padson the cylindrical inner surface, was fabricated as a prototype. Thematerials were selected to be minimally attenuating to MV therapeuticx-ray beams. In the example prototype, the dielectric support ring 120was formed from 10% fill polylactic acid (PLA) produced by a 3D printer,however other materials would be possible in a commercial version, e.g.,carbon fibre laminating a foam or hollow core. The conductive pads(shown black in the diagram) were formed from a very thin layer of aconductive paint (Bare Conductive, UK). The patient anatomy (e.g., heador other anatomy) would be located within the ring. As noted above,other non-ring geometries are possible, and the device could be made toadapt readily into existing mask immobilization systems commonly used inRT or SRS.

A computer was programmed to reads out the signals from all conductivepads (up to 12 at a time in the present example case), and calculateposition of the anatomy inside the ring. The position was showngraphically on a display device.

The specific embodiments described above have been shown by way ofexample, and it should be understood that these embodiments may besusceptible to various modifications and alternative forms. It should befurther understood that the claims are not intended to be limited to theparticular forms disclosed, but rather to cover all modifications,equivalents, and alternatives falling within the spirit and scope ofthis disclosure.

Therefore what is claimed is:
 1. A method of performing capacitivemonitoring the position of a body region during a medical procedureinvolving a therapeutic or surgical device, the method comprising:positioning the body region in a reference position associated with themedical procedure, wherein at least a portion of the body region ispositioned within a sensing region of a capacitive position sensingdevice, the capacitive position sensing device comprising an array ofelectrodes, and wherein the body region is positioned without contactingthe array of electrodes; detecting capacitance between each electrodeand the body region, thereby obtaining a set of measured capacitancevalues; processing the set of measured capacitance values to determinean offset of the body region relative to the reference position; and oneor more of: controlling the therapeutic or surgical device based on theoffset; and providing an alert to interrupt the medical procedure whenthe offset exceeds a threshold.
 2. The method according to claim 1wherein the offset comprises a positional offset.
 3. The methodaccording to claim 2 wherein the array of electrodes are arranged in athree-dimensional configuration, and wherein the positional offset ofthe body region is determined in three dimensions.
 4. The methodaccording to claim 2 wherein the processing of the set of measuredcapacitance values comprises: obtaining calibration data for eachelectrode of the array of electrodes, the calibration data establishinga relationship, for each electrode, between capacitance and positionaloffset; processing the set of measured capacitance values and thecalibration data to determine positional offsets that result in a set ofcalibration capacitance values for the array of electrodes that bestmatches the set of measured capacitance values.
 5. The method accordingto claim 4 wherein the calibration data is obtained from calibrationmeasurements of the body region obtained at a plurality of knownpositions relative to the reference position.
 6. The method according toclaim 4 wherein the calibration data is obtained from a plurality offinite element simulations, each finite element simulation comprising aset of capacitance values calculated at a unique spatial offset.
 7. Themethod according to claim 6 wherein the finite element simulations areperformed based on patient-specific model data.
 8. The method accordingto claim 7 wherein the model data is obtained from previously measuredvolumetric image data associated with the body region.
 9. The methodaccording to claim 2 wherein controlling the therapeutic or surgicaldevice comprises providing a signal to interrupt the operation of thetherapeutic or surgical device when the positional offset exceeds athreshold offset.
 10. The method according to claim 9 wherein thepositional offset comprises a plurality of positional offsets, whereineach positional offset is associated with a different spatial directionor dimension.
 11. The method according to claim 10 wherein eachpositional offset has a separate threshold offset associated therewith,and wherein the signal is provided to the therapeutic or surgical devicewhen one or more of the thresholds are exceeded.
 12. The methodaccording to claim 2 wherein controlling the therapeutic or surgicaldevice comprises providing the positional offset to the therapeutic orsurgical device such that the therapeutic or surgical device cancompensate for changes in the position of the body region.
 13. Themethod according to claim 2 wherein controlling the therapeutic orsurgical device comprises controlling a position and/or orientation ofthe therapeutic or surgical device to compensate for changes in theposition of the body region.
 14. The method according to claim 2 whereincontrolling the therapeutic or surgical device comprises providing thepositional offset to the therapeutic or surgical device or to a planningor navigation system associated with the therapeutic or surgical device,such that the location of one or more target locations associated withthe medical procedure are dynamically updated.
 15. The method accordingto claim 1 wherein the array of electrodes surrounds at least a portionof the body region.
 16. The method according to claim 1 wherein theelectrodes of the array of electrodes are arranged in a sphericalconfiguration.
 17. The method according to claim 1 wherein the array ofelectrodes comprises a first cylindrical array and at least oneadditional cylindrical array, wherein a first axis associated with thefirst cylindrical array and an additional axis associated with eachadditional cylindrical array are directed in different directions. 18.The method according to claim 1 wherein the therapeutic or surgicaldevice delivers an energy beam selected from the group consisting ofphoton energy beams, particle beams, and high-intensity focusedultrasound.
 19. The method according to claim 18 wherein at least aportion of the capacitive position sensing device is formed from amaterial having a thickness and composition suitable for transmitting atleast a portion of the energy beam during the medical procedure.
 20. Themethod according to claim 19 wherein the portion of the capacitiveposition sensing device comprises electrodes formed from a thin filmsuitable for transmitting at least 95 percent of the energy beam. 21.The method according to claim 18 wherein the capacitive position sensingdevice comprises an aperture configured to permit passage of the energybeam during the medical procedure, such that the energy beam is notoccluded by the capacitive position sensing device.
 22. The methodaccording to claim 1 wherein the reference position is defined at leastin part by a mask, and wherein the body region is placed adjacent to themask, and wherein the mask is formed from a dielectric material suchthat at least some of the capacitance values are measured through themask.
 23. The method according to claim 22 wherein one or both of themask and a supporting structure associated therewith forms at least aportion of the capacitive position sensing device, such that the maskand/or the supporting structure comprises the array of electrodes, andwherein at least one dielectric layer is present between each electrodeand the body region.
 24. The method according to claim 2 furthercomprising processing the set of measured capacitance values todetermine an angular offset of the body region about one or more axesrelative to the reference position; and controlling the therapeutic orsurgical device based on both the positional offset and the angularoffset.
 25. The method according to claim 1 wherein the offset comprisesan angular offset.
 26. A system for performing capacitive monitoring theposition of a body region during a medical procedure involving atherapeutic or surgical device, the system comprising: a capacitiveposition sensing device comprising: a dielectric support; and an arrayof electrodes provided on or embedded within said dielectric support;wherein said array of electrodes is configured for capacitive sensingwithin a sensing region, wherein the sensing region is suitable forpositioning at least a portion of the body region therein, such that thebody region is positionable in a reference position within the sensingvolume without contacting said array of electrodes; control andprocessing hardware operatively coupled to said capacitive positionsensing device, wherein said control and processing hardware isconnectable to said therapeutic or surgical device for sending a controlsignal thereto, and wherein said control and processing hardware isconfigured to perform operations comprising: detecting capacitancebetween each electrode and the body region, thereby obtaining a set ofmeasured capacitance values; processing the set of measured capacitancevalues to determine a positional offset of the body region relative tothe reference position; and at least one of: providing the controlsignal to the therapeutic or surgical device based on the positionaloffset; and providing an alert to interrupt the medical procedure whenthe positional offset exceeds a threshold.